Abstract
Clasts of shocked garnet-sillimanite gneisses comprise a minor fraction of the allochthonous breccia at the Haughton impact structure. Refractive indices of the diaplectic and fused components of the gneisses, and reduced specific gravity indicate shock pressures from 35 to 55±5 GPa and effective post-shock temperatures from 500° to 1,000° C in a suite of selected samples.
Sillimanites remain birefringent but display several effects of shock metamorphism. Shock-produced planar features and planar fractures are highly developed; optic axial angle (2V y ) increases from near normal (26°) to over 80° within a sample; there is a reduction in optical relief and a development of a pale brown colouring which generally deepens in shade as shock level increases. There is no unambiguous evidence, optically or from X-ray investigation, of a high-pressure Al2SiO5 polymorph or breakdown to mullite and silica. The highly shocked sillimanites have anomalous K2O contents from 0.11% to 0.92%. Potassium appears to substitute for aluminum and, to a lesser degree, for iron while retaining sillimanite stoichiometry, and the amount of substitution generally reflects increased shock level. The source of the contributed potassium is the coexisting shock-fused feldspar glass. The glass of each sample is derived primarily from melted alkali feldspar with a minor and varied admixture from the breakdown of mafic minerals. The glasses are depleted in K2O, although Na2O is unaffected, and the extent of depletion can be correlated with the increased K2O content of the associated sillimanite. The incorporation of potassium in shocked sillimanites is a function of both degree of shock deformation and availability of potassium from other coexisting shocked phases. It is speculated that the brown colouration is a function of ferrous iron content and may reflect post-crater thermal history rather than shock level.
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Contribution from the Earth Physics Branch No. 951
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Robertson, P.B., Plant, A.G. Shock metamorphism in sillimanite from the Haughton impact structure, Devon Island, Canada. Contr. Mineral. and Petrol. 78, 12–20 (1981). https://doi.org/10.1007/BF00371139
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DOI: https://doi.org/10.1007/BF00371139